PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 429 



The armature of the carapax, chelipeds, and ambulatory leg:s is i:iore 

 like ]\1. Bamffia thau tenuimana. There are usually six subequal and 

 nearly equidistant sjtines upon the anterior half of the lateral margin of 

 the carapax, of which one is in front of the cervical suture, three ui)on 

 the hepatic region, and two upon the anterior i)art of the branchial 

 region. There are no spines upon the posterior border of the carajiax 

 and none upon the abdomen, except two very small ones on the anterior 

 edge of the second somite. The chelipeds are very long and slender, iu 

 large specimens being a half or more longer than the entire body, 

 nearly cylindrical, and the merus and carpus sparsely armed with small 

 spines ; but the chela, which is longer, but no stouter, thau the merus, is 

 without spines. 

 Four specimens give the following measurements: 



LoDSth : 



Lt n^it h of carnpax, iniluding rostrum 



Length of rostrum .... 



Bruailth of cari^iax in front of cervical suture . 



Greatest bnnulih, excluding spiues 



Breadth near posterior margin 



Lnititli of cheliped 



L( iiiitli of merus 



Leni; t h of carpus 



L(ii;:th of cliela 



Li'iiirl h of dactylus 



Lengtli of first ambulatory leg. 



Greatest diameter of eye 



mm. 

 48.5 

 25.0 



9.1 

 11.1 

 13.7 

 12.3 

 U.O 

 34.3 



G.O 

 39.0 

 17.0 

 50.0 



4.0 



Tum,. 

 48.0 

 24.2 



8.9 

 11.7 

 14.0 

 13.0 

 79.0 

 32.0 



5.0 

 30. 

 17.4 

 47.7 



4.0 



mm, 



23.0 



12. 3 



4.9 



5.7 



G.2 



G.O 



36.5 



14.8 



3.0 



15.9 



7.2 



22.0 



3.0 



mm. 



17.5 

 9.2 

 3.4 

 4.4 

 4.8 

 4.5 



22.8 

 8.3 

 2.0 

 9.8 

 4.3 



14.0 

 2.0 



MACEUEA. 



Arctus depressus, sp. nov. 



This species is represented only by a single, small, and probably imma- 

 ture individual, but is apparently distinct from any known speci(\s of the 

 genus, and is readily distinguished by the very broad and greatly 

 depressed cephalo-thorax, which in these respects is like Ibacus, and by 

 the conspicuous spines each side of the posterior segments of the ster- 

 num. The depressed form is perhaps partially a character of imma- 

 turity, being- an ajiproach to the Plii/llosoma-stagefi, and it is possible 

 that the sternal spines disappear in the adult. 



The carapax is less than half as thick as broad, and the breadth is 

 much greater than the length along' the middle line above, but slightly 

 less than the length of the lateral margin, which is convex in outline, so 

 that the greatest breadth is near the middle of the length. The antero- 

 lateral angles are acute and very prominent, extending far forward of 

 the rest of the front and to a line slightly in advance of the lirst dor- 

 sally exposed segment (the true second peduncular) of the antenna each 

 side. The orbits are very large, altnost com])letely ojien iu front, and 

 occupy fully a third of the width of the whole front. The median carina 

 is low, being, even in the middle of its length, only a little higher than 

 the lateral carime, and rises into two low, dentiform prominences, one 



